Unilaterally mat, sealable, uv-stabilized, flame-resistant, co-extruded, biaxially oriented polyester film, method for the production and the use thereof

ABSTRACT

The invention concerns a unilaterally mat, sealable, UV-stabilized and flame-resistant, sealable, co-extruded, biaxially oriented polyester film comprising at least one base layer (B) and a sealable top coating (A) applied to both sides of said base layer, in addition to a mat covering (C). The film also contains at least one UV stabilizer as a light stabilizer. The invention further concerns a method for the production of said film and the use thereof.

[0001] Sealable, UV-resistant, flame-retardant, coextruded, biaxiallyoriented polyester film with one matt side, its use, and process for itsproduction

[0002] The invention relates to a sealable, UV-resistant andflame-retardant, coextruded, biaxally oriented polyester film with onematt side and composed of at least one base layer B and of, applied tothe two sides of this base layer, a sealable outer layer A and mattouter layer C. The film also comprises at least one light stabilizerwhich is a UV stabilizer, and comprises a flame retardant. The inventionalso includes the use of the film and a process for its production.

[0003] GB-A 1 465 973 describes a coextruded polyester film having twolayers, one layer of which consists of copolyesters containingisophthalic acid and terephthalic acid, and the other layer of whichconsists of polyethylene terephthalate. The patent gives no usefulindication of the sealing performance of the film. The lack ofpigmentation means that the film cannot be produced by a reliableprocess (cannot be wound up) and that the possibilities for furtherprocessing of the film are limited.

[0004] EP-A 0 035 835 describes a coextruded, sealable polyester filmwhere, in the sealable layer, particles whose average size exceeds thesealable layer thickness are present in order to improve winding andprocessing performance. The particulate additives form surfaceprotrusions which prevent undesired blocking and sticking of the film torolls or guides. No further details are given concerning theincorporation of antiblocking agents in relation to the other,nonsealable layer of the film. It is uncertain whether this layercomprises antiblocking agents. The choice of particles having diametersgreater than the sealable layer thickness, at the concentrations givenin the examples, impairs the sealing performance of the film. The patentdoes not give any indication of the sealing temperature range of thefilm. The seal seam strength is measured at 140° C. and is in the rangefrom 63 to 120 N/m (from 0.97 N/15 mm to 1.8 N/15 mm of film width).

[0005] EP-A 0 432 886 describes a coextruded multilayer polyester filmwhich has a first surface on which has been arranged a sealable layer,and has a second surface on which has been arranged an acrylate layer.The sealable outer layer here may also be composed ofisophthalic-acid-containing and terephthalic-acid-containingcopolyesters. The coating on the reverse side gives the film improvedprocessing performance. The patent gives no indication of the sealingrange of the film. The seal seam strength is measured at 140° C. For asealable layer thickness of 11 μm the seal seam strength given is 761.5N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coatingis that this side is now not sealable with respect to the sealable outerlayer, and the film therefore has only very restricted use.

[0006] EP-A 0 515 096 describes a coextruded, multilayer sealablepolyester film which comprises a further additive in the sealable layer.The additive may comprise inorganic particles, for example, and ispreferably distributed in an aqueous layer onto the film during itsproduction. Using this method, the film is claimed to retain its goodsealing properties and to be easy to process. The reverse side comprisesonly very few particles, most of which pass into this layer via therecycled material. This patent again gives no indication of the sealingtemperature range of the film. The seal seam strength is measured at140° C. and is above 200 N/m (3 N/15 mm). For a sealable layer of 3 μmthickness the seal seam strength given is 275 N/m (4.125 N/15 mm).

[0007] WO 98/06575 describes a coextruded, multilayer polyester filmwhich comprises a sealable outer layer and a nonsealable base layer. Thebase layer here may have been built up from one or more layers, and theinner layer of these layers is in contact with the sealable layer. Theother (outward-facing) layer then forms the second nonsealable outerlayer. Here, too, the sealable outer layer may be composed ofisophthalic-acid-containing and terephthalic-acid-containingcopolyesters, but these comprise no antiblocking particles. The filmalso comprises at least one UV absorber, which is added to the baselayer in a weight ratio of from 0.1 to 10%. The base layer has beenprovided with conventional antiblocking agents. The film has goodsealability, but does not have the desired processing performance andalso has shortcomings in optical properties. The film may also have onematt surface, but then has high haze, which is undesirable.

[0008] DE-A 23 46 787 describes a flame-retardant polymer. Besides thepolymer, its use to give films and fibers is also described. Thefollowing shortcomings were apparent during production of film usingthis claimed phospholane-modified polymer

[0009] The polymer mentioned is susceptible to hydrolysis and has to bevery effectively predried. When the polymer is dried using prior-artdryers it cakes, and production of a film is possible only under verydifficult conditions.

[0010] The films produced under uneconomic conditions also embrittle athigh temperatures, i.e. mechanical properties decline sharply as aresult of embrittlement, making the film unusable. This embrittlementarises after as little as 48 hours at high temperature.

[0011] Films which comprise no UV-absorbing materials exhibit yellowingand impairment of mechanical properties after even a short time inoutdoor applications, due to photooxidative degradation by sunlight.

[0012] Good mechanical properties include high modulus of elasticity(E_(MD)>3200 N/mm²; E_(TD)>3500 N/mm²) and also good values for tensilestress at break (in MD>100 N/mm²; in TD>130 N/mm²)

[0013] It was an object of the present invention to eliminate thedisadvantages of the prior art.

[0014] The invention provides a sealable, UV-resistant, flame-retardant,coextruded, biaxially oriented polyester film with one matt side andwith at least one base layer B, and one sealable outer layer A, andanother, matt, outer layer C, where at least one layer comprises a UVabsorber and a flame retardant, and where the sealable outer layer A hasa minimum sealing temperature of 110° C. and a seal seam strength of atleast 1.3 N/15 mm, and the topographies of the two outer layers A and Chave the following features:

[0015] Sealable outer layer A:

[0016] R_(a) value<30 nm

[0017] Value measured for gas flow from 500 to 4000 s

[0018] Nonsealable, matt outer layer C:

[0019] 200 nm<R_(a)<1000 nm

[0020] Value measured for gas flow<50 s.

[0021] The invention also provides the use of the film and a process forits production.

[0022] The sealable, transparent, UV-resistant, flame-retardant,coextruded, and biaxially oriented polyester film provided, with onematt side, does not therefore have the disadvantages of the prior-artfilms mentioned and in particular has good sealability, iscost-effective to produce, has improved processability, and has improvedoptical properties. In particular it has flame-retardant action and doesnot embrittle at high temperature.

[0023] The sealing range of the film has been extended to lowtemperatures, and the seal seam strength of the film has been increased,and at the same time the handling of the film has been improved over theprior art. It has also been ensured that the processability of the filmextends to high-speed processing machinery. Directly-arising regrind canbe reintroduced to the extrusion process during film production at aconcentration of up to 60% by weight, based on the total weight of thefilm, without any significant resultant adverse effect on the physicalproperties of the film.

[0024] Since the film of the invention is intended in particular foroutdoor applications and/or critical indoor applications, it is to havehigh UV resistance. High UV resistance means that sunlight or other UVradiation causes no, or only extremely little, damage to the films. Inparticular, when used outdoors for a period of some years the filmsshould not yellow or exhibit any embrittlement or surface cracking, norshow any impairment of mechanical properties. High UV resistancetherefore means that the film absorbs UV light and does not transmitlight until the visible region has been reached.

[0025] Flame-retardant action means that in what is known as a fireprotection test the transparent film complies with the conditions to DIN4102 Part 2 and in particular the conditions to DIN 4102 Part 1 and canbe allocated to construction materials class B2 and in particular B1 forlow-flammability materials.

[0026] The film is also intended to pass the UL 94 “Vertical BurningTest for Flammability of Plastic Materials”, permitting itsclassification as 94 VTM-0. This means that burning of the film hasceased 10 seconds after removal of the Bunsen burner, and that after 30seconds no smoldering is observed, and no burning drops are found tooccur.

[0027] Cost-effective production includes the capability of the rawmaterials or the raw material components needed to produce theflame-retardant film to be dried using prior-art industrial dryers. Itis significant that the raw materials do not cake and do not undergothermal degradation. The prior-art industrial dryers include vacuumdryers, fluidized-bed dryers, and fixed-bed dryers (tower dryers).

[0028] These dryers operate at temperatures from 100 to 170° C., atwhich the flame-retardant raw materials of the prior art used hithertogenerally cake and have to be removed by force, making film productionimpossible.

[0029] In vacuum dryers which have the gentlest drying action the rawmaterial passes through a range of temperature of from about 30 to 130°C. at a reduced pressure of 50 mbar. A process known as post-drying isthen required, in a hopper at temperatures of from 100 to 130° C. andwith a residence time of from 3 to 6 hours. Even here, the raw materialused hitherto cakes to an extreme extent.

[0030] No embrittlement on short-term exposure to high temperature meansthat after 100 hours of heat-conditioning at 100° C. in acirculating-air drying cabinet the film does not become brittle and doesnot have poor mechanical properties.

[0031] The UV stabilizer(s) is/are advantageously directly fed in theform of masterbatch(es) during film production, the concentration of theUV stabilizer(s) preferably being in the range from 0.01 to 5.0% byweight, with preference from 0.1 to 3.0% by weight, based on the weightof the relevant layer of the polyester used.

[0032] According to the invention, the film generally has at least threelayers, the layers then encompassed being the base layer B, the sealableouter layer A, and the matt outer layer C.

[0033] The base layer B of the film is generally composed of at least90% by weight of a thermoplastic polyester. Polyesters suitable for thispurpose are those made from ethylene glycol and terephthalic acid(polyethylene terephthalate, PET), from ethylene glycol andnaphthalene-2,6-dicarboxylic acid (polyethylene 2,6-naphthalate, PEN),from 1,4-bishydroxymethylcyclohexane and terephthalic acid(poly-1,4-cyclohexanedimethylene terephthalate, PCDT), or else made fromethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (polyethylene 2,6-naphthalatebibenzoate, PENBB). Particular preference is given to polyesters ofwhich at least 90 mol %, in particular at least 95 mol %, is composed ofethylene glycol units and terephthalic acid units, or of ethylene glycolunits and naphthalene-2,6-dicarboxylic acid units. The remaining monomerunits derive from other aliphatic, cycloaliphatic or aromatic diols and,respectively, dicarboxylic acids, as may also occur in the layers Aand/or C.

[0034] Other examples of suitable aliphatic diols are diethylene glycol,triethylene glycol, aliphatic glycols of the formula HO—(CH₂)_(n)—OH,where n is an integer from 3 to 6 (in particular 1,3-propanediol,1,4-butanediol, 1,5-pentanediol and 1,6-hexanediol) and branchedaliphatic glycols having up to 6 carbon atoms. Among the cycloaliphaticdials, mention should be made of cyclohexanediols (in particular1,4-cyclohexanediol). Examples of other suitable aromatic dials have theformula HO—C₆H₄—X—C₆H₄—OH, where X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—,—S— or —SO₂—. Bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also verysuitable.

[0035] Other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalene dicarboxylic acids (such asnaphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Among the cycloaliphatic dicarboxylic acids mention should bemade of cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Among the aliphatic dicarboxylicacids, the C₃-C₁₉ alkanediacids are particularly suitable, and thealkane moiety here may be straight-chain or branched.

[0036] One way of preparing these polyesters according to the inventionis the transesterification process. Here, the starting materials aredicarboxylic esters and dials, which are reacted using the customarytransesterification catalysts, such as the salts of zinc, of calcium, oflithium, of magnesium or of manganese. The intermediates are thenpolycondensed in the presence of well-known polycondensation catalysts,such as antimony trioxide or titanium salts. Another equally goodpreparation method is the direct esterification process in the presenceof polycondensation catalysts. This starts directly from thedicarboxylic acids and the diols.

[0037] The sealable outer layer A applied by coextrusion to the baselayer B is based on polyester copolymers and essentially consists ofamorphous copolyesters composed predominantly of isophthalic acid unitsand of terephthalic acid units, and of ethylene glycol units. Theremaining monomer units derive from other aliphatic, cycloaliphatic oraromatic diols and, respectively, dicarboxylic acids, as may also occurin the base layer. Preferred copolyesters providing the desired sealingproperties are those composed of ethylene terephthalate units andethylene isophthalate units. The proportion of ethylene terephthalate isfrom 40 to 95 mol %, and the corresponding proportion of ethyleneisophthalate is from 60 to 5 mol %. Preference is given to copolyestersin which the proportion of ethylene terephthalate is from 50 to 90 mol %and the corresponding proportion of ethylene isophthalate is from 50 to10 mol %, and particular preference is given to copolyesters in whichthe proportion of ethylene terephthalate is from 60 to 85 mol % and thecorresponding proportion of ethylene isophthalate is from 40 to 15 mol%.

[0038] The preferred embodiment of the matt outer layer C comprises ablend or a mixture made from two components I and II, and, whereappropriate, comprises additives in the form of inert inorganicantiblocking agents.

[0039] Component I of the mixture or of the blend is a polyethyleneterephthalate homopolymer or polyethylene terephthalate copolymer, or amixture made from polyethylene terephthalate homo- or copolymers.

[0040] Component II of the copolymer or of the mixture or of the blendis a polyethylene terephthalate copolymer composed of the condensationproduct of the following monomers or of their derivatives capable offorming polyesters:

[0041] A) from 65 to 95 mol % of isophthalic acid;

[0042] B) from 0 to 30 mol % of at least one aliphatic dicarboxylic acidhaving the formula HOOC(CH₂)_(n)COOH, where n is in the range from 1 to11;

[0043] C) from 5 to 15 mol % of at least one sulfomonomer containing analkali metal sulfonate group on the aromatic moiety of a dicarboxylicacid;

[0044] D) a copolymerizable aliphatic or cycloaliphatic glycol havingfrom 2 to 11 carbon atoms, in the stoichiometric amount needed to form100 mol % of condensate;

[0045] each of the percentages given being based on the total amount ofthe monomers forming component II. For a detailed description ofcomponent II see also EP-A-0 144 878, which is expressly incorporatedherein by way of reference.

[0046] For the purposes of the present invention, mixtures aremechanical mixtures prepared from the individual components. For this,the individual constituents are generally combined in the form ofsmall-dimensioned compressed moldings, e.g. lenticular or bead-shapedpellets, and mixed with one another mechanically, using a suitableagitator. Another way of producing the mixture is to feed components Iand II in pellet form separately to the extruder for the outer layer ofthe invention, and to carry out mixing in the extruder and/or in thedownstream systems for melt transportation.

[0047] For the purposes of the present invention, a blend is analloy-like composite of the individual components I and II which can nolonger be separated into the initial constituents. A blend hasproperties like those of a homogeneous material and can therefore becharacterized by appropriate parameters.

[0048] The ratio (by weight) of the two components I and II of the outerlayer mixture or of the blend can be varied within wide limits, anddepends on the intended use of the multilayer film. The ratio ofcomponents I and II is preferably in the range I:II=10:90 to I:II=95:5,preferably from I:II=20:80 to I:II=95:5, and in particular fromI:II=30:70 to I:II=95:5.

[0049] The desired sealing properties, the desired degree of mattness,and the desired processing properties of the film of the invention areobtained by combining the properties of the copolyester used for thesealable outer layer with the topographies of the sealable outer layer Aand of the nonsealable, matt outer layer C.

[0050] The minimum sealing temperature of 110° C. and the seal seamstrength of at least 1.3 N/15 mm are achieved when the copolymersdescribed in more detail above are used for the sealable outer layer A.The film has its best sealing properties when no other additives, inparticular no inorganic or organic fillers, are added to the copolymer.In this case, with the copolyester given above, the lowest minimumsealing temperature and the highest seal seam strengths are obtained.However, the handling of the film is poor in this case, since thesurface of the sealable outer layer A often tends to block. The film canhardly be wound and has little suitability for further processing onhigh-speed packaging machinery. To improve handling of the film, andprocessability, it is necessary to modify the sealable outer layer A.This is best done with the aid of suitable antiblocking agents of aselected size, which are added to the sealable layer at a particularconcentration, and specifically in such a way as to firstly minimizeblocking and secondly give only insignificant impairment of sealingproperties. This desired combination of properties can be achieved whenthe topography of the sealable outer layer A is characterized by thefollowing set of parameters:

[0051] The roughness of the sealable outer layer, characterized by theR_(a) value, is generally less than 30 nm, preferably less than 25 nm.Otherwise the sealing properties are adversely affected for the purposesof the present invention.

[0052] The value measured for gas flow should be from 500-4000 s,preferably from 600-3500 s. At values below 500 s the sealing propertiesare adversely affected for the purposes of the present invention, and atvalues above 4000 s the handling of the film becomes poor.

[0053] The nonsealable, matt outer layer C is characterized by thefollowing set of parameters

[0054] The roughness of the matt outer layer, characterized by its R_(a)value, is from 200 to 1000 nm, preferably from 220 to 900 nm. Valuessmaller than 200 nm have adverse effects on the winding and processingperformance of the film, and also on the degree of mattness of thesurface. Values greater than 1000 nm impair the optical properties(haze) of the film.

[0055] The value measured for gas flow should be in the range≦50 s,preferably≦45 s. The degree of mattness of the film is adverselyaffected at values above 50.

[0056] In principle, any of the organic or inorganic UV stabilizerssuitable for incorporation within polyesters may be selected. Thesesuitable UV stabilizers are known from the prior art, and examples aredescribed in more detail in WO 98/06575, EP-A-0 006 686, EP-A-0 031 202,EP-A-0 031 203, or EP-A-0 076 582.

[0057] UV stabilizers, i.e. light stabilizers which are UV absorbers,are generally chemical compounds which can intervene in the physical andchemical processes of light-induced degradation. Carbon black and otherpigments can give some protection from light. However, these substancesare unsuitable for transparent films, since they cause discoloration orcolor change. The only compounds suitable for transparent films arethose organic or organometallic compounds which produce no, or onlyextremely slight, color or color change in the thermoplastic to bestabilized.

[0058] Light stabilizers which are suitable UV stabilizers are thosewhich absorb at least 70%, preferably 80%, particularly preferably 90%,of the UV light in the wavelength region from 180 to 380 nm, preferablyfrom 280 to 360 nm. These are particularly suitable if they arethermally stable in the temperature range from 260 to 300° C., i.e. donot decompose and do not cause evolution of gas. Examples of lightstabilizers which are suitable UV stabilizers are2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickel compounds,salicylic esters, cinnamic ester derivatives, resorcinol monobenzoates,oxanilides, hydroxybenzoates, sterically hindered amines and triazines,preferably the 2-hydroxybenzotriazoles and the triazines.

[0059] In one preferred embodiment, the film of the invention comprisesfrom 0.01 to 5.0% by weight of2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol or from 0.01 to5.0% by weight of2,2′-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,2,2-tetramethylpropyl)phenolas UV-absorbing substance. It is also possible to use mixtures of thesetwo UV stabilizers, or mixtures of at least one of these two UVstabilizers with other UV stabilizers, the total concentration of lightstabilizer preferably being in the range from 0.01 to 5.0% by weight,based on the weight of the thermoplastic polyester.

[0060] The UV stabilizer(s) is/are advantageously metered in directly inthe form of masterbatch(es) during film production, the concentration ofthe UV stabilizer(s) preferably being from 0.01 to 5.0% by weight, withpreference from 0.1 to 3.0% by weight, based on the weight of therelevant layer of the polyester used.

[0061] In the three-layer embodiment, the UV stabilizer is preferablypresent in the nonsealable outer layer C. However, if required it isalso possible for the base layer B or even the sealable outer layer A tohave been provided with UV stabilizers. The concentration of thestabilizer(s) here is based on the weight of the thermoplastics in thelayer provided with UV stabilizer(s).

[0062] Surprisingly, weathering tests to the test specification ISO 4892using the Atlas Ci65 Weather-Ometer have shown that to improve UVresistance in the abovementioned three-layer film it is fully sufficientfor the outer layers of thickness from 0.3 to 2.5 μm to have beenprovided with UV stabilizers.

[0063] Weathering tests have shown that the UV-resistant films of theinvention do not generally show any yellowing or embrittlement or anyloss of surface gloss, nor show any surface cracking or impairment ofmechanical properties even in weathering tests extrapolated to from 5 to7 years of outdoor use.

[0064] The light stabilizer may be added before the material leaves thethermoplastic polymer producer or be fed into the extruder during filmproduction.

[0065] It is preferable to add the light stabilizer by way ofmasterbatch technology. For this, the additive(s) is/are firstcompletely dispersed in a carrier material. Carrier materials which maybe used are both the thermoplastic itself, e.g. the polyethyleneterephthalate, or else other polymers compatible with the thermoplastic.Once fed to the thermoplastic for film production, the constituents ofthe masterbatch melt during extrusion and are thus dissolved in thethermoplastic.

[0066] The concentration of the UV absorber alongside the thermoplasticin the masterbatch is from 2.0 to 50.0% by weight, preferably from 5.0to 30.0% by weight, the total of the constituents always being 100% byweight.

[0067] The film of the invention comprises at least one flame retardant,which is fed directly by way of the abovementioned masterbatchtechnology during film production, the concentration of the flameretardant being in the range from 0.5 to 30.0% by weight, preferablyfrom 1.0 to 20.0% by weight, based on the weight of the layer of thecrystallizable thermoplastic. The ratio maintained between flameretardant and thermoplastic when preparing the masterbatch is generallyin the range from 60:40 to 10:90% by weight.

[0068] Typical flame retardants include bromine compounds,chloroparaffins, and other chlorine compounds, antimony trioxide, andalumina trihydrates, but the halogen compounds are disadvantageous dueto the halogen-containing by-products produced. The low lightfastness offilms provided therewith is also extremely disadvantageous, as is theevolution of hydrogen halides in the event of a fire.

[0069] Examples of suitable flame retardants used according to theinvention are organophosphorus compounds, such as carboxyphosphinicacids, anhydrides thereof, and dimethyl methylphosphonate. It isimportant for the invention that the organophosphorus compound issoluble in the thermoplastic, since otherwise the optical propertiesdemanded are not achieved.

[0070] Since the flame retardants generally have some susceptibility tohydrolysis, concomitant use of a hydrolysis stabilizer may be advisable.

[0071] It was therefore more than surprising that the use of masterbatchtechnology and suitable predrying and/or precrystallization, and, whereappropriate, the use of small amounts of a hydrolysis stabilizer permitcost-effective production of a flame-retardant film with the requiredproperty profile with no caking in the dryer, and that the film does notembrittle on exposure to high temperature. Furthermore, neither anyevolution of gases nor any deposits were found during the productionprocess.

[0072] It was very surprising that together with this excellent resultand the required flame retardancy

[0073] within the limits of accuracy of measurement, there is no adverseeffect on the Yellowness Index of the film when comparison is made withan unprotected film;

[0074] there was no evolution of gases and there were no die depositsand no frame condensation, and the film therefore has excellent opticalproperties and excellent profile and layflat;

[0075] the flame-retardant film has excellent stretchability, and cantherefore be produced in a reliable and stable manner on high-speed filmlines at speeds of up to 420 m/min.

[0076] With this, the film is also cost-effective.

[0077] It is also very surprising that regrind produced from the filmsor from the moldings can be reused without any adverse effect on theYellowness Index of the film.

[0078] In one preferred embodiment, the film of the invention comprisesa crystallizable polyethylene terephthalate as main constituent, the UVabsorber, from 1 to 20% by weight of an organophosphorus compound asflame retardant soluble in the polyethylene terephthalate, and from 0.1to 1.0% by weight of a hydrolysis stabilizer.

[0079] Phenolic stabilizers, alkali metal/alkaline earth metalstearates, and/or alkali metal/alkaline earth metal carbonates areparticularly suitable. It is preferable for the phenolic stabilizers tobe used in amounts of from 0.05 to 0.6% by weight, in particular from0.15 to 0.3% by weight, and for their molar mass to be more than 500g/mol. Pentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate or1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4-hydroxybenzyl)benzene isparticularly advantageous.

[0080] In the three-layer embodiment, the flame retardant, like the UVabsorber, is preferably present in the nonsealable outer layer C.However, it is also possible if required for the flame retardants tohave been provided in the base layer B or even in the sealable outerlayer A. The concentration of the flame retardant(s) here is based onthe weight of thermoplastics in the layer provided with flameretardants.

[0081] Very surprisingly, fire protection tests to DIN 4102 and the ULtest have shown that to achieve improved flame retardancy in athree-layer film it is fully sufficient for the outer layers of from 0.3to 2.5 μm thickness to have been provided-with flame retardant. Ifrequired, and if fire protection requirements are stringent, it is alsopossible for the core layer to have been provided with flame retardant,i.e. to have what is known as a base level of provision.

[0082] The result is that when compared with monofilms provided withhigh concentrations throughout the flame-retardant multilayer filmsproduced by known coextrusion technology are of commercial interest,since markedly less flame retardant is needed.

[0083] Furthermore, measurements have shown that the film of theinvention does not embrittle over a prolonged period at a hightemperature of 100° C., and this is more than surprising. This result isattributable to the synergistic action of appropriateprecrystallization, predrying, masterbatch technology, and hydrolysisstabilizer.

[0084] The film is also readily recyclable without pollution of theenvironment and without loss of mechanical properties, making itsuitable for use as short-lived advertising placards, for example, or inconstruction of exhibition stands or for other promotional items, wherefire protection is desired.

[0085] Surprisingly, the compliance of films with construction materialsclasses B2 and B1 to DIN 4102 and with the UL 94 test extends to therange of thicknesses from 5 to 300 μm.

[0086] According to the invention, the flame retardant is added by wayof masterbatch technology. The flame retardant, where appropriate thehydrolysis stabilizer, and also the UV absorber, are completelydispersed as described above in a carrier material, and dissolve duringthe extrusion process.

[0087] An important factor in masterbatch technology is that theparticle size and the bulk density of the masterbatch are similar to theparticle size and bulk density of the thermoplastic, enablinghomogeneous dispersion and thus homogeneous UV resistance and flameretardancy.

[0088] It is important for the invention that the masterbatch whichcomprises the flame retardant and, where appropriate, a hydrolysisstabilizer, and a UV absorber is precrystallized or predried. Thispredrying includes, for example, gradual heating of the masterbatch atsubatmospheric pressure (from 20 to 80 mbar, preferably from 30 to 60mbar, in particular from 40 to 50 mbar) with agitation, and, if desired,post-drying at a constant, elevated temperature, again at subatmosphericpressure. It is preferable for the masterbatch to be charged at roomtemperature from a feed vessel in the desired blend together with thepolymers of the base and/or outer layers and, if desired, with other rawmaterial components batchwise into a vacuum dryer in which thetemperature profile traverses from 10 to 160° C., preferably from 20 to150° C., in particular from 30 to 130° C., during the course of thedrying time or residence time. During the residence time of about 6hours, preferably 5 hours, in particular 4 hours, the raw materialmixture is agitated at from 10 to 70 rpm, preferably from 15 to 65 rpm,in particular from 20 to 60 rpm. The resultant precrystallized orpredried raw material mixture is post-dried in a downstream vessel,likewise evacuated, at temperatures of from 90 to 180° C., preferablyfrom 100 to 170° C., in particular from 110 to 160° C., for from 2 to 8hours, preferably from 3 to 7 hours, in particular from 4 to 6 hours.

[0089] The base layer B may also comprise conventional additives, suchas stabilizers and/or antiblocking agents. The two other layers A and Cmay also comprise these additives. They are advantageously added to thepolymer or to the polymer mixture before melting begins. Examples ofstabilizers used are phosphorus compounds, such as phosphoric acid orphosphoric esters.

[0090] Suitable antiblocking agents (in this context also termedpigments) are inorganic and/or organic particles, such as calciumcarbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, aluminum oxide, LiF, the calcium,barium, zinc or manganese salts of the dicarboxylic acids used, carbonblack, titanium dioxide, kaolin or crosslinked polystyrene particles orcrosslinked acrylate particles.

[0091] The antiblocking agents selected may also be mixtures of two ormore different antiblocking agents or mixtures of antiblocking agents ofthe same composition but different particle size. The particles may beadded to the individual layers at the respective advantageousconcentrations, e.g. as a glycolic dispersion during thepolycondensation or by way of masterbatches during extrusion. Preferredparticles are SiO₂ in colloidal or in chain form. These particles becomevery well bound into the polymer matrix and create only very fewvacuoles. Vacuoles generally cause haze and it is therefore appropriateto avoid these. There is no restriction in principle on the diameters ofthe particles used. However, it has proven appropriate for achieving theobject to use particles with an average primary particle diameter below100 nm, preferably below 60 nm and particularly preferably below 50 nm,and/or particles with an average primary particle diameter above 1 μm,preferably above 1.5 μm and particularly preferably above 2 μm. However,the average particle diameter of these particles described last shouldnot be above 5 μm.

[0092] To achieve the abovementioned properties of the sealable film, ithas also proven to be appropriate to select a particle concentration inthe base layer B which is lower than in the two outer layers A and C. Ina three-layer film of the type mentioned, the particle concentration inthe base layer B will be from 0 to 0.15% by weight, preferably from0.001 to 0.12% by weight and in particular from 0.002 to 0.10% byweight. There is no restriction in principle on the diameter of theparticles used, but particular preference is given to particles with anaverage diameter above 1 μm.

[0093] In its advantageous usage form, the film of the invention iscomposed of three layers: the base layer B and, applied on both sides ofthis base layer, outer layers A and C, and outer layer A is sealablewith respect to itself and with respect to outer layer C.

[0094] To achieve the property profile mentioned for the-film, the outerlayer C has more pigment (i.e. a higher pigment concentration) than theouter layer A. The pigment concentration in this second, matt outerlayer C is from 1.0 to 10.0% by weight, advantageously from 1.5 to 10%by weight and in particular from 2.0 to 10% by weight. In contrast, theother outer layer A, which is sealable and positioned opposite to theouter layer C, has a lower degree of filling with inert pigments. Theconcentration of the inert particles in layer A is from 0.01 to 0.2% byweight, preferably from 0.015 to 0.15% by weight and in particular from0.02 to 0.1% by weight.

[0095] Between the base layer and the outer layers there may, ifdesired, also be an intermediate layer. This may again be composed ofthe polymers described for the base layers. In one particularlypreferred embodiment, it is composed of the polyester used for the baselayer. It may also comprise the customary additives described. Thethickness of the intermediate layer is generally above 0.3 μm,preferably in the range from 0.5 to 15 μm, in particular in the rangefrom 1.0 to 10 μm and very particularly preferably in the range from 1.0to 5 μm.

[0096] In the particularly advantageous three-layer embodiment of thefilm of the invention, the thickness of the outer layers A and C isgenerally above 0.1 μm, and is generally in the range from 0.2 to 4.0μm, advantageously in the range from 0.2 to 3.5 μm, in particular in therange from 0.3 to 3 μm and very particularly preferably in the rangefrom 0.3 to 2.5 μm, and the thicknesses of the outer layers A and C maybe identical or different.

[0097] The total thickness of the film of the invention may vary withinwide limits. It is from 3 to 100 μm, in particular from 4 to 80 μm,preferably from 5 to 70 μm, the layer B preferably making up from 5 to90% of the total thickness.

[0098] The polymers for the base layer B and the two outer layers A andC are introduced into three extruders. Any foreign bodies orcontamination present may be filtered out from the polymer melt prior toextrusion. The melts are then extruded through a coextrusion die to giveflat melt films, and layered one upon the other. The multilayer film isthen drawn off and solidified with the aid of a chill roll and, ifdesired, other rolls.

[0099] The film of the invention is generally produced by thecoextrusion process known per se.

[0100] The procedure for this process is that the melts corresponding tothe individual layers of the film are coextruded through a flat-filmdie, the resultant film is drawn off for solidification on one or morerolls, the film is then biaxially stretched (oriented), and thebiaxially stretched film is heat-set and, if desired, corona- orflame-treated on the surface layer intended for treatment.

[0101] The biaxial stretching (orientation) is generally carried outsequentially, and preference is given to sequential biaxial stretchingin which stretching is first longitudinal (in the machine direction) andthen transverse (perpendicular to the machine direction).

[0102] As is usual in coextrusion, the polymer or the polymer mixturefor the individual layers is first compressed and plasticized in anextruder, and the additives used may already be present in the polymeror the polymer mixture during this process. The melts are thensimultaneously extruded through a flat-film die (slot die), and theextruded multilayer film is drawn off on one or more take-off rolls,whereupon it cools and solidifies.

[0103] The biaxial orientation is generally carried out sequentially,preferably orienting first longitudinally (i.e. in the machinedirection=MD) and then transversely (i.e. perpendicularly to the machinedirection=TD). This gives orientation of the molecular chains. Thelongitudinal orientation can be carried out with the aid of two rollsrunning at different speeds corresponding to the desired stretchingratio. For the transverse orientation use is generally made of anappropriate tenter frame.

[0104] The temperature at which the orientation is carried out may varyover a relatively wide range and depends on the film properties desired.The longitudinal stretching is generally carried out at from about 80 to130° C., and the transverse stretching at from about 80 to 150° C. Thelongitudinal stretching ratio is generally in the range from 2.5:1 to6:1, preferably from 3:1 to 5.5:1. The transverse stretching ratio isgenerally in the range from 3.0:1 to 5.0:1, preferably from 3.5:1 to4.5:1. Prior to the transverse stretching, one or both surfaces of thefilm may be in-line coated by known processes. The in-line coating mayserve, for example, to give improved adhesion of a metal layer or of anyprinting ink applied, or else to improve antistatic performance orprocessing performance.

[0105] For producing a film with very good sealing properties it hasproven advantageous for the planar orientation Δp of the film to be lessthan 0.165, but particularly less than 0.163. In this case the strengthof the film in the direction of its thickness is so great that when theseal seam strength is measured it is specifically the seal seam whichseparates, and the tear does not enter the film or propagate therein.

[0106] The significant variables affecting the planar orientation Δphave been found to be the longitudinal and transverse stretchingparameters, and also the SV (standard viscosity) of the raw materialused. The process parameters include in particular the longitudinal andtransverse stretching ratios (λ_(MD) and λ_(TD)), the longitudinal andtransverse stretching temperatures (T_(MD) and T_(TD)), the film webspeed and the nature of the stretching, in particular that in thelongitudinal direction of the machine. For example, if the planarorientation Ap obtained with a machine is 0.167 with the following setof parameters: λ_(MD)=4.8 and λ_(TD)=4.0, a longitudinal stretchingtemperature T_(MD) of from 80-118° C. and a transverse stretchingtemperature T_(TD) of from 80-125° C., then increasing the longitudinalstretching temperature T_(MD) to 80-125° C. or increasing the transversestretching temperature T_(TD) to 80-135° C., or lowering thelongitudinal stretching ratio λ_(MD) to 4.3 or lowering the transversestretching ratio λ_(TD) to 3.7 gives a planar orientation Δp within thedesired range. The film web speed here is 340 m/min and the SV (standardviscosity) of the material is about 730. For the longitudinalstretching, the data mentioned are based on what is known as N-TEPstretching, composed of a low-orientation stretching step (LOE, towOrientation Elongation) and a high-orientation stretching step (REP,Rapid Elongation Process). Other stretching systems in principle givethe same ratios, but the numeric values for each process parameter maybe slightly different. The temperatures given are based on therespective roll temperatures in the case of the longitudinal stretchingand on infrared-measured film temperatures in the case of the transversestretching.

[0107] In the heat-setting which follows, the film is held for from 0.1to 10 s at a temperature of from 150 to 250° C. The film is then woundup in a usual manner.

[0108] After the biaxial stretching, it is preferable for one or bothsurfaces of the film to be corona- or flame-treated by one of the knownmethods. The intensity of the treatment is generally in the range above45 mN/m.

[0109] The film may also be coated in order to achieve other desiredproperties. Typical coatings are layers with adhesion-promoting,antistatic, slip-improving or release action. These additional layersmay, of course, be applied to the film by way of in-line coating, usingaqueous dispersions, prior to the transverse stretching step.

[0110] The film of the invention has excellent sealability, very goodflame retardancy, very good UV resistance, very good handling propertiesand very good processing performance. The sealable outer layer A of thefilm seals not only with respect to itself (fin sealing) but also withrespect to the nonsealable outer layer C (lap sealing). The minimumsealing temperature for the lap sealing here is only about 10 K higherthan the fin-sealing temperature, and the reduction in the seal seamstrength is not more than 0.3 N/15 mm.

[0111] It was also possible to improve the mattness of the film overprior-art films while at the same time reducing its haze. It has beenensured that regrind can be reintroduced to the extrusion process duringfilm production at a concentration of from 20 to 60% by weight, based onthe total weight of the film, without any significant resultant adverseeffect on the physical properties of the film.

[0112] The excellent sealing properties, very good handling propertiesand very good processing properties of the film make it particularlysuitable for processing on high-speed machinery.

[0113] The excellent combination of properties possessed by the film,furthermore, makes it suitable for a wide variety of differentapplications, for example for interior decoration, for the constructionof exhibition stands or for exhibition requisites, as displays, forplacards, for protective glazing of machinery or of vehicles, in thelighting sector, in the fitting out of shops or of stores, or as apromotional item or laminating medium.

[0114] The good UV resistance of the transparent film of the inventionmoreover makes it suitable for outdoor applications, e.g. forgreenhouses, roofing systems, exterior cladding, protective coveringsfor materials, e.g. for steel sheet, applications in the constructionsector, and illuminated advertising profiles, blinds, and electricalapplications.

[0115] The films and items produced from them are also particularlysuitable for outdoor applications where fire protection or flameretardancy is demanded.

[0116] The outer layer C has a characteristic matt, antireflectivesurface, and is therefore particularly attractive for the applicationsmentioned.

[0117] The table below (Table 1) gives once again the most importantproperties of the film of the invention. TABLE 1 Range accordingParticularly to the invention Preferred preferred Unit Test method Outerlayer A Minimum sealing temperature <110 <105 <100 ° C. internal Sealseam strength >1.3 >1.5 >1.8 N/15 mm internal Average roughness R_(a)<30 <25 <20 nm DIN 4768, cut-off 0.25 mm Range of values measured for500-4000  800-3500 1000-3000 sec internal gas flow Gloss,20° >120 >130 >140 DIN 67 530 Outer layer C COF <0.5 <0.45 <0.40 DIN 53375 Average roughness R_(a) 200-1000 225-900 250-800 nm DIN 4768,cut-off 0.25 mm Range of values measured for <50 <45 <49 sec internalgas flow Gloss, 60° <60 <55 <50 DIN 67 530 Other film properties Haze<40 <35 <30 % ASTM-D 1003-52 Planar orientation <0.1650 <0.163 <0.160internal Fire performance The film meets the requirements forconstruction materials classes B2 and B1 to DIN 4102 Part 2/Part 1 andpasses the UL 94 test Weathering test, UV <20% ISO 4892 resistanceChange in properties¹⁾

[0118] Each of the properties in the examples below was measured inaccordance with the following standards or methods.

[0119] SV (DCA), IV (DVE)

[0120] Standard viscosity SV (DCA) is measured in dichloroacetic acid bya method based on DIN 53726. Intrinsic viscosity (IV) is calculated asfollows from the standard viscosity

IV (DCA)=6.67·10⁻⁴ SV·(DCA)+0.118

[0121] Determination of Minimum Sealing Temperature

[0122] Hot-sealed specimens (seal seam 20 mm×100 mm) are produced with aBrugger HSG/ET sealing apparatus by sealing the film at differenttemperatures with the aid of two heated sealing jaws at a sealingpressure of 2 bar and with a sealing time of 0.5 s. From the sealedspecimens test strips of 15 mm width were cut. T-seal seam strength wasmeasured as in the determination of seal seam strength. The minimumsealing temperature is the temperature at which a seal seam strength ofat least 0.5 N/15 mm is achieved.

[0123] Seal Seam Strength

[0124] To determine seal seam strength, two film strips of width 15 mmwere placed one on top of the other and sealed at 130° C. with a sealingtime of 0.5 s and a sealing pressure of 2 bar (apparatus: Brugger modelNDS, single-side-heated sealing jaw). Seal seam strength was determinedby the T-peel method.

[0125] Coefficient of Friction

[0126] Coefficient of friction was determined to DIN 53 375. Thecoefficient of sliding friction was measured 14 days after production.

[0127] Surface Tension

[0128] Surface tension was determined by what is known as the ink method(DIN 53 364).

[0129] Haze

[0130] Hölz haze was measured by a method based on ASTM-D 1003-52 but,in order to utilize the most effective measurement range, measurementswere made on four pieces of film laid one on top of the other, and a 1°slit diaphragm was used instead of a 4° pinhole.

[0131] Gloss

[0132] Gloss was determined to DIN 67 530. The reflectance was measuredas an optical value characteristic of a film surface. Based on thestandards ASTM-D 523-78 and ISO 2813, the angle of incidence was set at20°. A beam of light hits the flat test surface at the set angle ofincidence and is reflected and/or scattered thereby. A proportionalelectrical variable is displayed representing light rays hitting thephotoelectronic detector. The value measured is dimensionless and mustbe stated together with the angle of incidence.

[0133] Surface Gas Flow Time

[0134] The principle of the test method is based on air flow between oneside of the film and a smooth silicon wafer sheet. The air flows fromthe surroundings into an evacuated space, and the interface between filmand silicon wafer sheet acts as a flow resistance. A round specimen offilm is placed on a silicon wafer sheet in the middle of which there isa hole providing the connection to the receiver. The receiver isevacuated to a pressure below 0.1 mbar. The time in seconds taken by theair to establish a pressure rise of 56 mbar in the receiver isdetermined. Test conditions: Test area 45.1 cm² Weight applied 1276 gAir temperature 23° C. Humidity 50% relative humidity Aggregated gasvolume 1.2 cm³ Pressure difference 56 mbar

[0135] Determination of Planar Orientation Δp

[0136] Planar orientation is determined by measuring the refractiveindices with an Abbe refractometer, using internal operatingspecification 24.

[0137] Preparation of Specimens

[0138] Specimen size and length: from 60 to 100 mm

[0139] Specimen width: corresponds to prism width of 10 mm

[0140] To determine n_(MD) and n_(α) (=n_(z)), the specimen to be testedhas to be cut out from the film with the running edge of the specimenrunning precisely in the direction TD. To determine n_(MD) and n_(α)(=n_(z)), the specimen to be tested has to be cut out from the film withthe running edge of the specimen running precisely in the direction MD.The specimens are to be taken from the middle of the film web. Care mustbe taken that the temperature of the Abbe refractometer is 23° C. Usinga glass rod, a little diiodomethane (N=1.745) ordiiodomethane-bromo-naphthalene mixture is applied to the lower prism,which has been cleaned thoroughly before the test. The refractive indexof the mixture must be greater than 1.685. The specimen cut out in thedirection TD is firstly laid on top of this, in such a way that theentire surface of the prism is covered. Using a paper wipe, the film isnow firmly pressed flat onto the prism, so that it is firmly andsmoothly positioned thereon. The excess liquid must be sucked away. Alittle of the test liquid is then dropped onto the film. The secondprism is swung down and into place and pressed firmly into contact. Theright-hand knurled screw is then used to turn the indicator scale untila transition from light to dark can be seen in the field of view in therange from 1.62 to 1.68. If the transition from light to dark is notsharp, the colors are brought together using the upper knurled screw insuch a way that only one light and one dark zone are visible. The sharptransition line is brought to the crossing point of the two diagonallines (in the eyepiece) using the lower knurled screw. The value nowindicated on the measurement scale is read off and entered into the testrecord. This is the refractive index n_(MD) in the machine direction.The scale is now turned using the lower knurled screw until the rangevisible in the eyepiece is from 1.49 to 1.50. The refractive index n_(α)or n_(z) (in the direction of the thickness of the film) is thendetermined. To improve the visibility of the transition, which is onlyweakly visible, a polarization film is placed over the eyepiece. This isturned until the transition is clearly visible. The same considerationsapply as in the determination of n_(MD). If the transition from light todark is not sharp (colored), the colors are brought together using theupper knurled screw in such a way that a sharp transition can be seen.This sharp transition line is placed on the crossing point of the twodiagonal lines using the lower knurled screw, and the value indicated onthe scale is read off and entered into the table.

[0141] The specimen is then turned, and the corresponding refractiveindices n_(MD) and n_(α) (=n_(z)) of the other side are measured andentered into an appropriate table.

[0142] After determining the refractive indices in, respectively, thedirection MD and the direction of the thickness of the film, thespecimen strip cut out in the direction MD is placed in position and therefractive indices n_(MD) and n_(α) (=n_(z)) are determined accordingly.The strip is turned over, and the values for the B side are measured.The values for the A side and the B side are combined to give averagerefractive indices. The orientation values are then calculated from therefractive indices using the following formulae:

Δn=n _(MD) −n _(TD)

Δp=(n _(MD) +n _(TD))/2−n _(z)

n _(av)=(n _(MD) +n _(TD) +n _(z))/3

[0143] Surface Defects

[0144] Surface defects are determined visually.

[0145] Mechanical Properties

[0146] Modulus of elasticity, tear strength and elongation at break aremeasured longitudinally and transversely to ISO 527-1-2.

[0147] Weathering (on Both Sides), UV Resistance

[0148] UV resistance is tested as follows to the test specification ISO4892 Test apparatus: Atlas Ci65 Weather-Ometer Test conditions: ISO4892, i.e. artificial weathering Irradiation time: 1000 hours (per side)Irradiation: 0.5 W/m², 340 nm Temperature: 63° C. Relative humidity: 50%Xenon lamp: inner and outer filter made from borosilicate Irradiationcycles: 102 minutes of UV light, then 18 minutes of UV light with waterspray on the specimens, then again 102 minutes of UV light, etc.

[0149] Color Change

[0150] The change in color of the specimens after artificial weatheringis measured using a spectrophotometer to DIN 5033.

[0151] The greater the numerical deviation from standard, the greaterthe color difference. Numerical values of 0.3 can be neglected andindicate that there is no significant color change.

[0152] Yellowness

[0153] Yellowness Index YID is the deviation from the colorlesscondition in the “yellow” direction, and is measured to DIN 6167.Yellowness values<5 are not visually detectable.

[0154] Fire Performance

[0155] Fire performance is determined to DIN 4102 Part 2, constructionmaterials class B2 and to DIN 4102 Part 1, construction materials classB1, and also by the UL 94 test.

EXAMPLES

[0156] The examples below and the comparative examples in each case usefilms of varying thickness, produced by a known extrusion process.

Example 1

[0157] Chips made from polyethylene terephthalate (prepared by thetransesterification process with Mn as transesterification catalyst, Mnconcentration: 100 ppm) were dried at 150° C. to residual moisture below100 ppm and fed to the extruder for the base layer B. Chips made frompolyethylene terephthalate and from a filler were likewise fed to theextruder for the nonsealable outer layer C.

[0158] Alongside this, chips were prepared made from a linear polyesterwhich is composed of an amorphous copolyester having 78 mol % ofethylene terephthalate and 22 mol % of ethylene isophthalate (preparedvia the transesterification process with Mn as transesterificationcatalyst, Mn concentration: 100 ppm). The copolyester was dried at atemperature of 100° C. to residual moisture below 200 ppm and fed to theextruder for the sealable outer layer A.

[0159] The UV stabilizer2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol (®Tinuvin 1577) isfed in the form of masterbatches. The masterbatches are composed of 5%by weight of Tinuvin 1577 as active component and 95% by weight ofpolyethylene terephthalate (for outer layer C) and, respectively, 95% byweight of polyethylene isophthalate (for outer layer A). The 5% byweight strength Tinuvin 1577 is fed only to the two thick outer layers,20% by weight of the respective masterbatches by way of masterbatchtechnology.

[0160] The hydrolysis stabilizer and the flame retardant are fed in theform of a masterbatch. The masterbatch is composed of 20% by weight offlame retardant, 1% by weight of hydrolysis stabilizer, and 79% byweight of polyethylene terephthalate. The hydrolysis stabilizer ispentaerythrityltetrakis-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate. The flameretardant is dimethyl methyl-phosphonate (®Armgard P 1045). The bulkdensity of the masterbatch is 750 kg/m³ and its softening point is 69°C.

[0161] The masterbatch is charged at room temperature from a separatemetering vessel into a vacuum dryer, which from the time of charging tothe end of the residence time traverses a temperature profile of from 25to 130° C. During the residence time of about 4 hours, the masterbatchis stirred at 61 rpm. The precrystallized or predried masterbatch isafter-dried in the downstream hopper, likewise in vacuo, at 140° C. for4 hours.

[0162] 10% by weight of the masterbatch are added to the base layer B,and 20% by weight of the masterbatch are added to the nonsealable outerlayer C.

[0163] Coextrusion, followed by stepwise longitudinal and transverseorientation, is used to produce a transparent three-layer film with ABCstructure and with a total thickness of 12 μm. The thickness of eachouter layer can be found in Table 2.

[0164] Outer layer A is a mixture made from:

[0165] 20.0% by weight of Uw masterbatch based on polyethyleneisophthalate

[0166] 77.0% by weight of copolyester with an SV of 800

[0167] 3.0% by weight of masterbatch made from 97.75% by weight ofcopolyester (SV of 800) and 1.0% by weight of ®Sylobloc 44 H (syntheticSiO₂ from Grace) and 1.25% by weight of ®Aerosil TT 600 (fumed SiO₂ fromDegussa)

[0168] Base layer B:

[0169] 90.0% by weight of polyethylene terephthalate with an SV of 800

[0170] 10.0% by weight of masterbatch which comprises flame retardantand hydrolysis stabilizer

[0171] Outer layer C is a mixture made from:

[0172] 20.0% by weight of masterbatch which comprises flame retardantand hydrolysis stabilizer

[0173] 20.0% by weight of UV masterbatch based on polyethyleneterephthalate

[0174] 45.0% by weight of polyethylene terephthalate with an SV of 800(=component I)

[0175] 15.0% by weight of component II

[0176] Component II was prepared as described in more detail in EP-A-0144 878.

[0177] The production conditions in the individual steps of the processwere: Extrusion: Temperatures A layer: 270° C. B layer: 290° C. C layer:290° C. Die gap width: 2.5 mm Take-off roll Temperature: 30° C.Longitudinal Temperature: 80-125° C. stretching: Longitudinal 4.2stretching ratio: Transverse Temperature: 80-135° C. stretching:Transverse 4.0 stretching ratio: Heat-setting: Temperature: 230° C.Duration: 3 s

[0178] The film had the required good sealing properties, the desiredmattness, and exhibits the desired handling properties and the desiredprocessing performance. The film structure and the properties achievedin films prepared in this way are given in Tables 2 and 3.

[0179] The films in this example, and in all of the examples below, wereweathered on both sides, in each case for 1000 hours per side, using theAtlas Ci 65 Weather-Ometer to test specification ISO 4892 and thentested for mechanical properties, discoloration, surface defects, hazeand gloss.

[0180] The film meets the requirements of construction materials classesB2 and B1 to DIN 4102 Part 2 and Part 1. The film passes the UL 94 test.

Example 2

[0181] In comparison with Example 1, the outer layer thickness of thesealable layer A was raised from 1.5 to 2.0 μm. This has given improvedsealing properties, and in particular the seal seam strength hasincreased markedly.

Example 3

[0182] In comparison with Example 1, the film produced now had athickness of 20 μm. The outer layer thickness for the sealable layer Awas 2.5 μm and that for the nonsealable layer C was 2.0 μm. This hasagain improved sealing properties, and in particular the seal seamstrength has increased markedly, and the handling properties of the filmhave improved slightly.

Example 4

[0183] In comparison with Example 3, the copolymer for the sealableouter layer A has been changed. Instead of the amorphous copolyesterwith 78 mol % of polyethylene terephthalate and 22 mol % of ethyleneterephthalate, use was now made of an amorphous copolyester with 70 mol% of polyethylene terephthalate and 30 mol % of ethylene terephthalate.The polymer was processed in a twin-screw vented extruder, without anyneed for predrying. The outer layer thickness for the sealable layer Awas again 2.5 μm, and that for the nonsealable layer C was 2.0 μm. Thishas given improved sealing properties, and in particular the seal seamstrength has increased markedly. To achieve good handling properties andgood processing performance from the film, the pigment concentration inthe two outer layers was raised slightly.

Comparative Example 1

[0184] In comparison with Example 1, the sealable outer layer A was nownot pigmented. Although this has given some improvement in the sealingproperties, the handling properties of the film and its processingperformance have worsened unacceptably.

Comparative Example 2

[0185] In comparison with Example 1, the level of pigmentation in thesealable outer layer A was now as high as in the nonsealable outer layerC. This measure has improved the handling properties and the processingproperties of the film, but the sealing properties have worsenedmarkedly.

Comparative Example 3

[0186] In comparison with Example 1, the nonsealable outer layer A wasnow pigmented to a markedly lower level. The handling properties of thefilm and its processing performance have worsened markedly.

Comparative Example 4

[0187] Example 1 from EP-A 0 035 835 was repeated. The sealingperformance of the film, its handling properties and its processingperformance are poorer than in the examples according to the invention.TABLE 2 Average pigment Pigment Film Layer thicknesses diameterconcentrations thickness Film μm Pigments in layers in layers μm ppmExample μm Structure A B C A B C A B C A B C E 1 12 ABC 1.5 9 1.5Sylobloc 44 H none 0 2.5 2.5 300 0 0 Aerosil TT 600 0.04 0.04 375 E 2 12ABC 2.0 8.5 1.5 Sylobloc 44 H none 0 2.5 2.5 300 0 0 Aerosil TT 600 0.040.04 375 E 3 20 ABC 2.5 15.5 2.0 Sylobloc 44 H none 0 2.5 2.5 300 0 0Aerosil TT 600 0.04 0.04 375 E 4 20 ABC 2.5 15.5 2.0 Sylobloc 44 H none0 2.5 2.5 400 0 0 Aerosil TT 600 0.04 0.04 500 CE 1 12 ABC 1.5 9 1.5none none Sylobloc 44 H 2.5 0 1200 Aerosil TT 600 0.04 1500 CE 2 12 ABC1.5 9 1.5 Sylobloc 44 H none Sylobloc 44 H 2.5 2.5 300 0 1200 Aerosil TT600 Aerosil TT 600 0.04 0.04 375 1500 CC 3 12 ABC 1.5 9 1.5 Sylobloc 44H none Sylobloc 44 H 2.5 2.5 300 0 600 Aerosil TT 600 Aerosil TT 6000.04 0.04 375 750 CE 4 15 AB 2.25 12.75 Gasil 35 none 3 2500 0 EP-A 035835

[0188] TABLE 3 Coeffi- Minimum cient of sealing Seal seam frictionAverage temperature strength COF roughness Values Winding ° C. A side Cside R_(a) measured perform- Process- A side with with with nm for gasflow Gloss ance and ing respect to A respect to respect to A C A C A Chandling perform- Example side A side C side side side side side Δp sideside Haze properties ance E 1 100 2.0 0.45 25 340 1200 20 0.165 140 5032 ++ ++ E 2 98 2.7 0.45 26 340 1280 20 0.165 140 50 32 ++ ++ E 3 95 3.00.41 23 340 1110 20 0.165 130 45 34 ++ ++ E 4 85 3.3 0.40 23 340 1300 200.165 130 45 34 ++ ++ CE 1 98 2.1 0.45 10 65 10,000 80 0.165 160 170 1.5− − CE 2 110 1.0 0.45 65 65 80 80 0.165 130 170 2.8 − − CE 3 100 2.00.45 25 37 1200 150 0.165 160 190 1.5 − − CE 4 115 0.97 >2 70 2050 >5000 12 − −

[0189] TABLE 4 Total Modulus of elasticity Tensile stress at breakTensile strain at break dis- Weather- N/mm² N/mm² % coloration SurfaceGloss Example ing longitudinal transverse longitudinal transverselongitudinal transverse value defects A side C side Haze 1 Before 43005800 220 280 170 100 140 170 2.5 After 4100 5480 190 270 150 90 0.2 none132 165 2.8 2 Before 4200 5600 215 260 170 100 140 170 2.5 After 40305400 190 250 150 90 0.25 none 138 165 2.8 3 Before 4500 5700 230 280 175105 130 170 3.0 After 4000 5350 196 255 150 89 0.24 none 138 155 3.7 4Before 4300 5800 220 275 178 111 130 170 3.0 After 3900 5360 192 248 14892 0.27 none 138 165 3.5

1. A sealable, UV-resistant, flame-retardant, coextruded, biaxiallyoriented polyester film with one matt side and with at least one baselayer B based on a thermoplastic polyester, and one sealable outer layerA, and one other, matt, outer layer C, where at least one layercomprises a UV absorber and also a flame retardant, wherein the sealableouter layer A has a minimum sealing temperature of 110° C. and a sealseam strength of at least 1.3 N/15 mm, and the topographies of the twoouter layers A and C have the following features Sealable outer layer A:R_(a) value<30 nm Value measured for gas flow from 500 to 4000 sNonsealable, matt outer layer C: 200 nm<R_(a)<1000 nm Value measured forgas flow<50 s.
 2. The film as claimed in claim 1, wherein the sealableouter layer A comprises an amorphous copolyester which is composed ofethylene terephthalate units and of ethylene isophthalate units and ofethylene glycol units.
 3. The film as claimed in claim 1 or 2, whereinthe amorphous copolyester of the sealable outer layer A contains from 40to 95 mol % of ethylene terephthalate and from 60 to 5 mol % of ethyleneisophthalate, preferably from 50 to 90 mol % of ethylene terephthalateand from 50 to 10 mol % of ethylene isophthalate, and particularlypreferably from 60 to 85 mol % of ethylene terephthalate and from 40 to15 mol % of ethylene isophthalate.
 4. The film as claimed in one or moreof claims 1 to 3, wherein the matt outer layer C comprises a blend or amixture made from two components I and II, and, where appropriate,comprises additives in the form of inert inorganic antiblocking agents.5. The film as claimed in one or more of claims 1 to 4, wherein theconcentration of the UV absorber is in the range from 0.01 to 5.0% byweight, preferably from 0.1 to 3% by weight, based on the weight of therespective layer of the polyester used.
 6. The film as claimed in one ormore of claims 1 to 5, which comprises, as UV absorbers,2-hydroxybenzophenones, 2-hydroxybenzotriazoles, organonickel compounds,salicylic esters, cinnamic ester derivatives, resorcinol monobenzoates,oxanilides, hydroxybenzoic esters, sterically hindered amines and/ortriazines, preferably 2-hydroxybenzotriazoles and triazines, and inparticular 2-(4,6-diphenyl-1,3,5-triazin-2-yl)-5-hexyloxyphenol or2,2′-methylenebis(6-(2H-benzotriazol-2-yl)-4-(1,1,2,2-tetramethylpropyl)phenol.
 7. Thefilm as claimed in one or more of claims 1 to 6, wherein theconcentration of the flame retardant is in the range from 0.5 to 30.0%by weight, preferably from 1.0 to 20.0% by weight, based on the weightof the respective layer of the polyester used.
 8. The film as claimed inone or more of claims 1 to 7, wherein the flame retardant used comprisesorganophosphorus compounds, preferably carboxyphosphinic acids,anhydrides of these, or dimethyl methylphosphonate.
 9. The film asclaimed in one or more of claims 1 to 8, wherein regrind is present at aconcentration of up to 60% by weight, based on the total weight of thefilm.
 10. A process for producing a sealable, UV-resistant,flame-retardant, biaxially oriented polyester film with one matt sideand with at least one base layer B based on a thermoplastic polyester,and one sealable outer layer A, and one other, matt, outer layer C,where at least one layer comprises a UV absorber and a flame retardantadded by way of masterbatch technology, and where the masterbatch hasbeen precrystallized and/or predried, which comprises coextruding,through a flat-film die, the melts corresponding to each of the layersof the film, drawing off the resultant film on one or more rolls forsolidification, and then biaxially stretching (orienting) the film, andheat-setting the biaxially stretched film.
 11. The process as claimed inclaim 10, wherein the film is corona- or flame-treated on the surfaceintended for treatment.
 12. The process as claimed in claim 10 or 11,wherein the UV absorber is added by the producer of the thermoplasticpolymer or during film production into the extruder, addition by way ofmasterbatch technology being preferred.
 13. The process as claimed inclaim 10, wherein the flame retardant is present in the masterbatch witha ratio of flame retardant to thermoplastic in the range from 60:40% byweight to 10:90% by weight.
 14. The process as claimed in claim 10 or11, wherein in the masterbatch use is also made of a hydrolysisstabilizer in the form of phenolic stabilizers, alkali metal/alkalineearth metal stearates, and/or alkali metal/alkaline earth metalcarbonates, in amounts of from 0.05 to 0.6% by weight, preferably from0.15 to 0.3% by weight, and with a molar mass of more than 500 g/mol.15. The use of the film as claimed in one or more of claims 1 to 9 forindoor or outdoor applications.
 16. The use as claimed in claim 15indoors for interior decoration, for the construction of exhibitionstands or for exhibition requisites, as displays, for placards, forprotective glazing of machinery or of vehicles, in the lighting sector,in the fitting out of shops or of stores, as a promotional item, orlaminating medium, or outdoors for greenhouses, in the advertisingsector, roofing systems, exterior cladding, protective coverings formaterials, applications in the construction sector, or illuminatedadvertising profiles.